1. Field of the Invention
The present invention relates to volumetric flow measurements and more particularly to in vivo blood flow measurements of cardiac output.
2. Description of the Prior Art
Blood flow measurement is an important diagnostic technique used in various medical applications. A prime application is in the diagnostic analysis and treatment of heart conditions, where accurate volumetric blood measurement can be extremely useful, particularly since the physical dimensions of the vessels containing the moving blood cannot be reliably measured. Another important reason for measuring blood flow over a period of time is to determine the effect of medication on cardiac output.
Because of the importance of measuring blood flow in humans, much attention and effort has been devoted to developing a reliable blood flow measurement technique. A primary desire is to determine the blood flow exiting from the heart. It is well known to those of ordinary skill in the art that several physiological factors have been particularly difficult to overcome. For example, because the flow distribution of blood from the heart is non-stationary and dynamic, measuring blood flow at a peripheral artery does not provide an accurate estimate by extrapolation of primary heart blood flow. Moreover, measuring blood flow at the heart implies that the site of the flow measurement is the pulmonary artery or the aortic root, both of which are relatively inaccessible to a measuring device, although the pulmonary artery is more easily accessed at the bedside than the aorta.
Another physiologic factor which has not previously been fully appreciated is that the main pulmonary artery is relatively short and very elastic, lacking rigidity in all directions. In the clinical environment, the cross-sectional area varies significantly from person to person, and even moment to moment in the same person, depending upon blood volume, drugs, posture, disease states, and even blood flow itself. Any measurement technique which requires assumptions about vessel cross-sectional area is therefore likely to fail in a clinical environment. Thus, as used herein, flow is the measurement of mass fluid movement and relates to the volume of fluid moved per unit time. Knowledge of fluid velocity or velocity profiles is not required, nor is information regarding the cross-sectional area of the flow chamber.
Generally, there are three prior art approaches to measuring blood flow in the pulmonary artery. One way, an invasive procedure, is to mount one of several types of transducers on a flow-directed catheter, insert the catheter through a large peripheral vein such as the superior vena cava, and float it into the heart. A comprehensive review of such techniques for using and inserting pulmonary artery catheters has been given by Keefer et al. in Chapter 9 of a text entitled "Monitoring in Anesthesia and Critical Care Medicine," pp. 177-228, and thus will not be described here. Once in the heart, the transducer can measure the flow of the blood passing it. Velocity can be measured using ultrasound Doppler techniques or by estimating the time of flight between two points on the catheter.
An example of the latter type of system for measuring cardiac output is described in U.S. Pat. No. 4,542,748 to Roy. Such time of flight measurements occasionally utilize stochastic or cross-correlation techniques; however, since they do not estimate cross-sectional area, the ultimate blood flow measurement has an inherent error. Ultrasound techniques, on the other hand, may not only measure blood flow by using the Doppler effect, but also may estimate a cross-sectional area by measuring a vessel diameter as in U.S. Pat. No. 4,733,669 to Segal. In particular, Segal discloses a catheter having a mechanism for positioning a Doppler shift transducer carried by a catheter against a side wall of a blood vessel for accurately monitoring blood flow on the right side of the heart. However, the technique disclosed by Segal provides unacceptably inaccurate data since the vessel is not a perfect circle and hence its cross-sectional area cannot reliably be estimated merely by measuring a diameter. Moreover, as explained above, the pulmonary artery is very short and hence small changes in catheter position also add a component to the error in the flow estimate.
A second method of measuring blood flow is to mount an ultrasound transducer on a vehicle which will lie in a structure surrounding the great vessels, such as the esophagus or trachea, measure flow, and estimate vessel cross-section. An example of such a method is disclosed in U.S. Pat. No. 4,722,347 to Abrams et al. Abrams et al. disclose a non-invasive ultrasound apparatus which is inserted into the trachea in proximity to the aorta or pulmonary artery. Ultrasound waves are transmitted toward the path of flow of blood in the artery and reflected waves are received. The average Doppler frequency difference between transmitted and received waves is measured for determining the cross-sectional size of the artery as well as blood flow rate. Abrams et al further disclose in "Transtracheal Doppler: A New Procedure for Continuous Cardiac Output Measurement," Anesthesiology, Vol. 70, No. 1, January, 1989, pp 134-138, that such techniques lead to favorable cardiac output measurements in dogs. However, the methodology used by Abrams et al. dictates that the performance of such devices is still very dependent upon the user's ability to obtain an accurate estimate of the vessel cross-sectional area. Thus, for the reasons pointed out above, these techniques are also unsatisfactory in a clinical environment.
In addition, ultrasound transducers have been placed adjacent to but external to the vascular walls, as in DATASCOPE.TM. Company's "ARTERSOUND".TM. system. In this device, two ultrasound transducers are used. One transducer is placed in the esophageal to measure the velocity of the descending aorta, while the other is placed over the sternal notch to measure the aortic root dimensions. Again, as with the devices previously mentioned, the accuracy and precision of such a device is poor. Moreover, significant user experience is needed to obtain the measurements.
Finally, a third method of measuring blood flow is by indicator dilution techniques. Multiple methods of indicator introduction have been taught in the prior art. One such method is to introduce the indicator proximally according to a predetermined function and to measure the function attenuation distally. For example, waveforms may be introduced either as a step function as disclosed in U.S. Pat. Nos. 4,217,910 and 4,240,441 to Khalil, in "Measurement of Cardiac Output by Thermal-Dilution and Direct Fick Methods in Dogs," J. Appl. Physiol. 21(3):I 1131-35 (1966) by Khalil et al., and in "A Continuous Cardiac Output Computer Based on Thermodilution Principles," Annals of Biomedical Engineering, Vol. 17, pp 61-73, 1989 by Normann et al., or as a series of sine waves as disclosed in U.S. Pat. No. 4,236,527 to Newbower et al. and U.S. Pat. No. 4,380,237 to Newbower, and in "Continuous Thermal Measurement of Cardiac Output," IEEE Trans. on Biomed. Engin., Vol. BME-31, No. 5, May, 1984, pp. 393-400 to Philip et al. The respective references disclose the use of a thermodilution catheter which introduces heat or some other indicator into the blood stream to be propagated downstream and detected. The data collected is then processed to determine blood flow without knowledge of the vessel geometry, for indicator dilution techniques generally do not require knowledge of vessel geometry to determine volumetric flow. However, the techniques of the above-mentioned references are less than satisfactory since in many applications they may require extensive calibration procedures or may be contaminated by background thermal noise.
Thermal noise results from the fact that the temperature of the blood in the superior vena cava is generally higher than that returning from the lower extremities though the inferior vena cava. In other words, the amount and temperatures of such returning blood are not constant and therefore produce variations in pulmonary artery temperature. A primary source of such thermal variation in the temperature of blood within the heart results from respiration. For example, when a breath is taken, the proportion of blood entering the heart from the superior and inferior venae cavae varies, thus varying the temperature of the resulting mixture in the pulmonary artery. Ventilation itself has little or no effect on the temperature of the blood in the pulmonary artery, since the cooling effects of gas exchange occur downstream from that location.
In addition, because blood vessel cross-sectional area is so difficult to reliably estimate, successful clinical methods of measuring blood flow must utilize a technique which does not require knowledge of vessel dimensions. Investigators such as Bassingthwaighte and Rosenkrantz have demonstrated that vascular systems are linear and time invariant, thus permitting classical system identification techniques to be used. Such techniques are described by Bassingthwaighte et al. in "Applications of the Lagged Normal Density Curve as a Model for Arterial Dilution Curves," Circulation Research, Vol. 18, pp. 398-407, April 1966, and by Rosenkrantz et al. in "Pseudorandom Noise and Cross-Correlation in Indicator-Dilution Systems,"Journal of Surgical Research, Vol. 21, pp. 105-11 (1976). The clinical standard, using an indicator dilution method as taught by Stewart in "The Output of the Heart in Dogs," American Journal of Physiology, Vol. 57, pp. 27-50 (1921) is essentially a conversation of mass/heat technique. For example, when using dye, heat, cold, or other indicator, a bolus may be injected into the proximal end of a vessel and its appearance measured at a distal point. Thus, because the indicator is generally conserved, measuring the distal indicator's appearance and knowing the amount injected allows calculation of the true bulk mass blood flow. This technique is further described by Barankay et al. in "Cardiac Output Estimation by a Thermodilution Method Involving Intravascular Heating and Thermistor Recording," Acta Physiologica Academiae Scientiarium Hungericae, Tomus 38(2-3), pp. 167-173 (1970) and by Normann et al. Unfortunately, however, these techniques cannot be safely used with a heat indicator since the application of a "heat bolus" would produce high temperature at the surface of the heater element which may damage blood cells and circulatory connective or muscle tissues. Moreover, such a technique is time consuming and provides only intermittent determinations.
An improvement to the above techniques was provided by U.S. Pat. No. 4,507,974 to the present inventor, which is fully incorporated above by reference. The Yelderman '974 patent incorporates two basic ideas, namely, stochastic system identification and conservation of mass equations. Bassingthwaighte and Rosenkrantz substantiated the system identification principles and Stewart and others verified conservation of mass principles. Yelderman '974 was the first to put the two together by teaching that any indicator may be introduced in the form of any stochastic or spread spectral process. In particular, in that system a catheter mounted heating filament utilizes a stochastic or pseudorandom input to drive the heating element. Such an input causes a continuous low level excitation (nonimpulsive) waveform to be possible which allows for a physiologically safe surface temperature at the heater filament, unlike other systems which require large amounts of peak energy. The vascular system impulse response is then measured downstream and cross-correlated with the input signal. This information is combined with the conservation of heat equation to measure volumetric fluid flow. An advantage of this technique is that the total impulse response function of the vascular tree may be accurately estimated, which is not possible in the prior art system of Newbower, for example, and may be estimated with a low peak power. Knowing the complete response improves performance in the presence of disturbances and eliminates the requirement to calibrate the procedure. Also, the impulse response measurement is combined with a conservation of mass/heat equation that underlies the thermodilution measurement to allow calculation of the true volumetric blood flow. Such measurement is relatively accurate and depends neither upon knowledge nor estimation of vessel cross-sectional area.
However, at present, no thermal filament type based blood flow measuring device has yet been developed or approved for commercial sale in the United States, for when attempts have been made in the past to apply them in clinical settings, they have not proven sufficiently accurate to displace classical injected indicator techniques. Thus, although the technique disclosed by Yelderman provides performance superior to any other known thermal technique in a noisy environment, some improvement is desirable for use in actual clinical settings where the degree of thermal noise is quite high.
Therefore, a long felt need exists to provide an automatic technique of continuously measuring blood flow which does not require knowledge of the vessel cross-section, but which also eliminates or substantially reduces the background noise inherent in the flow system so that the thermodilution flow measurement technique may be adapted to the clinical setting. The present invention has been designed to meet these needs.